1. Field of the Invention
The present invention relates generally to a method of and apparatus for producing a thin uniform film or layer, such as a magnetic thin film, a metallic thin film, and a dielectric thin film, on a substrate by sputtering. More particularly, the present invention relates to an opposed target type sputtering apparatus adapted for producing a thin film having a uniform thickness on a substrate, especially a continuously conveyed wide substrate, and suitable for obtaining a magnetic recording medium or a transparent electroconductive member adapted to be used for the manufacture of electric and electronic parts. The opposed target type sputtering apparatus is characterized in that the sputtering surfaces of the opposed targets can be effectively used for producing a thin uniform film on a substrate by sputtering, and that, even if a substrate is wide, the thickness of the film on the wide substrate can be easily controlled.
2. Description of the Related Art
A typical opposed target type sputtering apparatus is disclosed in Japanese Unexamined patent publication No. 57-158380, in which a pair of cathode targets are opposedly arranged in a vacuum vessel, and a magnetic field is generated in the same direction as the opposing direction of the targets. A substrate is arranged at the side of the opposed targets so that a film is produced on the substrate by sputtering. The opposed target type sputtering apparatus is suitable for rapidly producing a film of magnetic material under a low temperature, and thus the apparatus is adopted for the manufacture of a magnetic thin film and a magnetic recording medium.
However, as reported in IEEE Transactions on Magnetics MAG-17, 1981, Pg. 3175, when the above-described opposed target type sputtering apparatus is used for continuously producing a thin film, such as a Co-Cr alloy film of a perpendicular magnetic recording medium, the central part of the sputtering surfaces of the opposed targets is locally and concentrically eroded. Therefore, the efficiency in use of the sputtering surface of the opposed targets is necessarily low. In addition, an uneven thickness of the film occurs in the direction of the width of a substrate on which the film is deposited, and accordingly, the conventional sputtering apparatus is not suitable for a large-scale production of a film by sputtering.
To overcome these problems encountered by the above-described conventional sputtering apparatus, the present inventors proposed a different conventional opposed target type sputtering apparatus in Japanese Unexamined Patent Publications Nos. 58-164781 and 59-116376. The proposed opposed target type sputtering apparatus has a construction such that magnetic core members forming a part of the magnetic field generating unit are arranged around the opposed targets, as will be understood from FIG. 17 illustrating a portion of the proposed opposed target type sputtering apparatus. In FIG. 17, the core members 301 and 302 are arranged around the opposed targets T and T'. The core members 301 and 302 have end portions 301a and 302a inclined and extending toward the surfaces of respective targets T and T' so as to be capable of acting as a magnetic core as well as a shield to prevent electron bombardment. The core members 301 and 302 also have leg portions 301b and 302b around which magnetic field generating sources 301' and 302' consisting of solenoids or permanent magnets are disposed to be magnetically connected to the leg portions 301b and 302b; Thus, a magnetic field H is concentrically generated in the region surrounding the targets T and T'. The sputtering apparatus also has a vacuum vessel 310, target holders 311 and 312, and coolant conduits 311a and 312a. According to the arrangement of the magnetic core members 301 and 302, the magnetic field H is generated directly between the magnetic core members 301 and 302 without passing through the targets T and T'. Therefore, the distribution of the magnetic field is not affected by the magnetic permeability and saturated magnetization of the material of the targets or the thickness of the targets, and thus is always stable. Further, since the magnetic field H for capturing or confining plasma is generated around the targets, the eroded area of the sputtering surfaces of the targets radially extends from the central portion toward the periphery of the sputtering surfaces of the targets. Accordingly, the efficiency in use of the sputtering surfaces of the targets is high. However, when the sputtering takes place, an electric discharge voltage appearing between the opposed targets T and T' is very high, and therefore, the production of the sputtered film cannot be achieved at a high production rate unless a high sputtering gas pressure is formed in the vessel of the apparatus. However, due to the high sputtering gas pressure, the opposed target type sputtering apparatus cannot be permitted to exhibit the typical features thereof, and thus a control of a required crystal structure and a required crystallite texture of the sputtered film cannot be easily achieved. Moreover, when the width of a substrate on which a sputtered film is to be deposited is large, and accordingly, when the width of the respective targets is large, the difference in thickness of the produced film between the central portion and the end portions of the substrate in the direction of the width of the substrate becomes large, and progress of the sputtering erosion of the targets in the central portion thereof is quicker than that in the other portions. As a result, the efficiency in use of the targets is reduced. The present inventors have, therefore, conducted research and experiments to obtain an improved opposed target type sputtering apparatus, and thus have become aware that their research should be directed to the cooperation of the shields (the anodes) of the sputtering apparatus with the perpendicular magnetic field for confining the .gamma.-electrons. According to the present inventors' experiments, it was clarified that the shields of the sputtering apparatus tend to absorb electrons (Y-electrons and thermoelectrons) in the plasma, and that this absorption of the electrons causes the afore-mentioned problem of the opposed target type sputtering apparatus. Therefore, the present inventors contrived to arrange reflecting electrodes, which carry out an operation contrary to that of the above-mentioned shields, in the orbiting region of the electrons controlled by the magnetic field generating unit disposed around the outer periphery of each of the opposed targets, so that the electrons are reflected by the reflecting electrodes. As a result, it was confirmed that the electric discharge characteristic is much improved and that a film having an excellent quality can be produced under a low gas pressure and a low electric voltage. It was also confirmed that, when an auxiliary magnetic field having a field component in parallel with the target surfaces for capturing the electrons is generated along the outer peripheries of the targets and adjacent to the front faces of the targets, the above-mentioned electric discharge characteristic can be further improved, and that the eroded area is extensively increased, i.e., local concentration of the eroded area to the central portion of the targets can be eliminated. As a result, it was confirmed that a uniform erosion of the entire sputtering surfaces of the targets, conventionally considered impossible, can be achieved.